Posted
by
ScuttleMonkey
on Monday February 09, 2009 @03:34PM
from the high-tech-to-the-lowest-bid dept.

pha7boy writes "The Herschel space observatory, the European Space Agency's answer to the Hubble Telescope, is about to be sent into orbit. With a mirror 1.5 times the size of the Hubble mirror, the Herschel will look at the universe in the infrared and sub-millimeter range. This 'will permit Herschel to see past the dust that scatters Hubble's visible wavelengths, and to gaze at really cold places and objects in the Universe — from the birthing clouds of new stars to the icy comets that live far out in the Solar System.'"

I think Hubble has been one of the most interesting and successful space based missions ever. A lot of the most mind blowing images I've ever seen of space have come from that telescope. Hopefully this telescope will continue the trend.

A lot of progress has taken place in the the field of optics, electronics, cryogenics, material science and communications. Given the additional 1m on the reflector, it'd be safe to assume a far better performance than Hubble.

Given the additional 1m on the reflector, it'd be safe to assume a far better performance than Hubble.

Yep. Meters matter. A lot. The summary says the mirror is 1.5 times as big (3.5m/2.4m), but really area and thus quantity of incident light is what matters so it's more like 210% as big as Hubble (3.5^2)/(2.4^2). This is a big space telescope. All else being the same, I'd expect this to show a good deal more distant/faint objects.

It says it's infrared, so this may be more comparable to Spitzer than Hubble. Spitzer is only 0.85m. This beast is 17 times the light bucket Spitzer is.

Diffraction limit is set by lambda/diameter -- the longer the wavelength to larger the mirror required to get the same limit.

But then again all I know about telescopes comes from hobbyist visible light scopes.

On the ground you're almost always resolution limited by the atmospheric seeing, as long as your aperture is over ~10cm or so. Anything larger form there is going to help with collecting more light (i.e. detecting fainter objects with shorter exposures) but not with resolution any more.

In a certain respect, it is related to the focal length, if you consider the eyepiece or sensor fixed. If you have a fixed pixel size on your CCD, then changing the focal length will change the angular size of each pixel, and thus change the resolution, although I think this kind of result is usually called magnification. Similarly, when using an eyepiece, the magnification is related to the ratio of the focal lengths, so a longer focal length will change the magnification

When the CCD pixels get small enough though, that size is no longer the limit on the resolution. Instead (neglecting atmospheric effects) you run into the fact that photon impacts are defined by probability densities that behave like waves, and you get a certain 'spreading' around the nominal impact location. The diameter of this spreading (the Airy disk) means that two sources that are too close together cannot be distinguished from each other, and this is called the diffraction limit. (There are other equally valid explanations for this effect, particularly coming from a wave perspective, this is just the one that I started typing.)

Now, in order to reduce this you want to bring in more photons from further seperated distances, meaning you want a larger aperture in order to improve the diffraction limited resolution. Generally the limiting angular resolution is given by theta_r = 1.4 lambda/D. Of course, if you have too strong of aberrations in the optical system, have to deal with atmospheric 'seeing' effects, the system is not diffraction limited, and the point spread function spreads out more.

Of course, its dangerous to compare the capabilities of telescopes at different wavelengths (Hubble is visible, Herschel is infrared to millimeter wave), because the total amount of light available changes, the angular resolution changes, and the engineering requirements change. Really, Hubble is about the maximum size optical space telescope you can make with current launch vehicles without moving to a completely new kind of telescope (active feedback with wavefront sensing like JWST). Herschel is able to be bigger easily because it requires significantly lower precision, due to the larger wavelengths.

Due note too, that the James Webb Space Telescope (the US's next telescope slated to launch in 2013, assuming funding doesn't dry up) is slated to have a 6.5m mirror, which should produce some REALLY nice results.

In addition to that, the JWST will feature a micro-shutter array, composed of over 62,000 individual shutters in the area of a postage stamp. The idea is that each shutter can be independently opened and closed so visible light from near, bright objects can be blocked out making it easier to view objects that are further away.

I had the opportunity to tour the clean room at Goddard Space Flight Center where the array is being fabricated. The techniques used seemed to mirror the techniques used to manufacture modern microprocessors. It was very interesting to be guided through the process, there are definitely some incredibly smart people at Goddard.

True, but JWST and Herschel are looking at vastly different wavelengths of light (mostly visible and near-IR as compared to submillimeter) so they really compliment each other. JWSTis a replacement for Hubble in the truest sense of the word. The article is misleading in it's claim that Herschel is a replacement for Hubble.

If you'd like a preview, check out the results from BLAST [blastexperiment.info] (more results and even prettier pictures coming out very soon). Although it only has a 1.8 meter mirror, it has the same version of detectors that the SPIRE instrument on Herschel uses. To be cheaper and faster, BLAST flys on a high-altitude balloon platform. Slashdot has covered it [slashdot.org] in the past. And there's a documentary [blastthemovie.com] about BLAST as well (also covered [slashdot.org] by slashdot).

As described in here [esa.int], the point of putting the observatory in a Lissajous orbit [wikipedia.org] around the Earth-Sun L2 Lagrange point [wikipedia.org] is to have the three nearest and largest sources of infrared light pollution (the earth, the moon, and the sun) sufficiently far away and in the same hemisphere relative to the observatory, allowing for a clear viewing angle anywhere in the other hemisphere.

This instrument is capable of great science, but spatial resolving power is not it's strong suit. Since it is measuring at wavelenghts much greater than hubble (100-1000x), the 3.5 meter mirror doesn't give you anything like hubble.

As for all the new discoverys i'm sure these new telescopes will find, i'm curious if they will do the same thing with these as they did with the hubble, by pointing it at a "black" region of space and leaving it there for a while gathering exposures, only to discover that the region wasn't "black" at all, it was completely filled with all sorts of different galaxies, and this was only a small point in space they were looking.

Astronomers were well aware that the region of sky used for the Hubble deep field was full of distant objects. It was chosen because the lack of bright, close objects makes it simpler to do really long exposures of the distant, dim ones.

One of the stated goals of the Herschel is to look back to the beginnings of the universe. The best way to do that is by choosing a dark area of the sky and exposing as long as you can.

Astronomers were well aware that the region of sky used for the Hubble deep field was full of distant objects.

Wow, what a bunch of historical revisionism. How, exactly, were astronomers supposed to be 'well aware' that that pointing Hubble at a patch of sky where all previous surveys had shown nothing at all, would produce such interesting results?

What actually happened was that the director of the observatory has some 'discretionary' observing time on the telescope, that he/she is completely free to do

Bravo. The post full of true but irrelevant detail, the tone of righteous indignation... excellent troll.

Just in case you were serious, have a read through the wikipedia entry [wikipedia.org] for the Hubble deep field, and the original paper [harvard.edu] (PDF link).

Pay particular attention to the "Field Selection" section of the paper. Note that both the Wikipedia article and the introduction to the paper talk about how the project was designed to image a "typical" field, with the intention of studying (among other things) galaxy morphology in the very early universe. Also, the introduction to the paper outlines several other preceding medium and deep field studies that provided the motivation for the Hubble deep field project (goes to how the astronomers knew they were likely to find galaxies - previous studies did NOT show nothing at all).

Now, moving on to the field selection part of the paper, note how the target field was chosen because of the absence of nearby sources, and upper limits on the strength of the sources present (not NO sources present, but rather no STRONG, CLOSE sources present).

The kicker of course, is this quote: "Eisenhardt (1995) kindly provided KPNO 4m R-band CCD images (2x300 second exposures) as further verification that the fields were typical in terms of source counts..." (emphasis mine). A truly empty portion of the sky, with no observable signal sources would be VERY atypical.

Taking a quick tour through the source table in the paper, I noticed a source at around magnitude 17. The tables later in the paper suggested that there are sources present up to magnitude 15. That's pretty faint, but magnitude 15 is reachable by moderate amateur telescopes and the naked eye. Considerably better can be done with amateur CCD equipment and long exposures.

One of the fundamental assumptions of cosmology is that, on the large scale, the universe is pretty much the same no matter what direction you look. A totally blank area back as far as Hubble can see would be very unusual.

Again, I'm sorry to ruin your beautiful troll with actual references, but it was so well done someone might believe you!

PS: the "a normal application... would have been rejected" claim doesn't seem to be nearly as iron-clad as your tone implied.

"The landmark research was carried out under Williams' direction, and using a significant fraction of his own director's discretionary time on the Space Telescope. He decided to conduct the Hubble Deep Field program to use Space Telescope's exquisite resolution and high sensitivity

Did you read back to the beginning of the thread? A poster wondered if the Herschel would be used to do a deep field, like Hubble, and seemed to be under the impression that the presence of galaxies in the Hubble deep field was a surprising discovery. I mentioned that it was not, then the poster you're referring to began his post with:

Wow, what a bunch of historical revisionism. How, exactly, were astronomers supposed to be 'well aware' that that pointing Hubble at a patch of sky where all previous survey

My point is that they _were_ aware (or next to 100% certain) that there would be distant objects as the current theory says that we can not see the edge of the known universe in any direction. Everything else (barring technical defects, etc) would be an even greater discovery.

And that is without the facts (no, I did not verify them myself) ceoyoyo gave us.

In fact, there is no case for very long exposures like the Hubble Deep Field or the Chandra Deep Fields (X-rays) with infrared telescopes, because the maximum depth reachable is not limited by sensitivity (exposure time) but by confusion (resolving power).The confusion limit is reached when you can not detect any more sources because the field is so crowded that they start overlapping with each other. This limit is usually reached in infrared telescopes long before the detection limit (a few minutes), because the wavelength of the light in this spectral range is so big that the resolving power is very poor.Note that resolving power is proportional to the diameter of the main mirror and inversely proportional to the observing wavelength, so a ~4.5m telescope like Herschel operating at 100 microns has aproximately half resolving power of an amateur 6cm telescope operating in visible light.This also implies that the "ESA response to Hubble" statement is absurd and misleading

A telescope with a bigger mirror can concentrate more light, therefore it sees fainter, more distant, objects. And the further away things are in the universe, the more red-shifted their light is. It really makes sense a space telescope being designed for infrared light, rather than visible.

Thanks for the info. One thing that confuses me about that is assuming everything is moving away from the origin of the universe, wouldn't all galaxies always move away from each other. I recall reading that in some many billions of years, another galaxy will collide with ours. Wouldn't these 2 outcomes be mutually exclusive? Genuinely curious about this.

However, if you do the math, you'll see that even the earliest possible galaxies (redshift = z = 10 (or less)) don't get visible light redshifted into the submillimeter (where Herschel looks). This is because the earliest galaxies are very dusty, and this dust obscures most of the visible light coming from the stars. However, to obscure the light, the dust is absorbing the energy, heating the dust up (to about 35K give or take) and this dust re-radiates at like 10-100 microns or so, which is redshifted in

It depends what you're comparing. Hubble looks a bit into the near IR too. Spitzer is mostly mid-IR, and Herschel is designed to look at very long wave IR and the higher frequency microwave region. Herschel and Spitzer overlap in wavelength a little bit, but not really that much.

In terms of application, Spitzer is not in the same sensitivity class as Hubble or Herschel, so for really deep field imaging the comparison between Hubble and Herschel is fairly apt.

And it's not really a Spitzer replacement, as it's looking at mostly longer wavelengths than Spitzer. It complements both. Virtually all different wavelengths are worth looking at as they all tells us unique things.

Herschel, mostly with the SPIRE instrument, will be looking at the earliest star forming galaxies. A nearly identical instrument to SPIRE was flown on a high-altitude balloon, BLAST [blastexperiment.info], which got Slashdot [slashdot.org] coverage [slashdot.org] as well.

When you're looking at things really really far away, the frequencies shift towards the red end of the spectrum due to the doppler effect of the Hubble Expansion. If we only looked in the visible spectrum, we wouldn't see anything, because the light had already shifted out of the proper range. Thus, but looking towards the infrared and longer wavelengths, we can actually detect things that originally light emitted in the visible spectrum but are reaching us in a heavily stretched state.

A lot of these objects are really far away, and over the billions of years since the light was emitted, the actual space that its had to travel has grown as the universe expands. All of that adds up and the light is significantly red-shifted.

True. But the oldest galaxies (what Herschel is mainly designed to look at) don't emit in the visible, even in their own (rest) frame. That's because the earliest galaxies are very dusty, and all this dust is opaque to the visible light. The stars are still there, glowing away, but their light is absorbed by this dust. This absorption heats the dust, warming it to 35K (give or take), which, as all things with non-zero temperature do, emits radiation like a blackbody. This light is then redshifted such

Early galaxies dusty? How? I mean, where did the dust come from? Isn't dust heavier elements, formed in stars, so in early universe and early galaxies, there would have been much less dust than in current galaxies?

I wasn't exactly sure myself until this comment [slashdot.org] and this wiki entry [wikipedia.org]. If we focused them on visible spectrums, we'd not notice the most distant emissions. Since attempting to detect obejects that are extremely distant is the apparently the whole bit with the Herschel telescope it starts to make sense.

That still doesn't answer the question about why they didn't include the visible spectrum. The way I see it, if you're building this giant space telescope with multiple sensors anyway, why not throw in a visible light camera for good measure? Sure, it may not be the main focus of the mission, but when Hubble's electronics finally bite the dust (and at this rate, that may be before too long), there's going to be a real lack of these breathtaking images of space for many years. All of that could be avoided

Because an optical design for IR will not work well in the visible. It's not as simple as slapping an Si detector somewhere in the field and snapping images. The optical prescription of the telescope has to be tailored to the operating wavelength.

I assume you're referring to chromatic aberration? Bear in mind that the James Webb satellite I mentioned already detects the red and possibly orange portions of the visible spectrum. If you're not getting too much chromatic aberration there, then even in the worst case, I'd expect the amount you'd get in the visible spectrum to be minor enough to be correctable in software. It should be easily correctable with an additional lens, with a modified sensor design, or with some combination of those two plus

I was being quick in my reply and not very exact. Sorry for the confusion. You are correct that the chromatic aberrations do not matter until you get into the refractive components of the individual science instruments.

Assuming that the reflectivity of the surfaces is still good at visible wavelengths, the part of the prescription that changes are the surface figure requirements and tolerancing (spacing, alignment, etc.)

Because the radiation emitted by stars isn't just visible light. There's all sorts of EM waves being transmitted. By gathering a range of EM waves, instead of just visible light, we can gather information, which is critical when you think of the tiny numbers of photons we receive from these distant stars.

I'm only an amateur astronomer but... With adaptive optics we can get better visible light images with ground based telescopes like Keck than with any orbiting telecope that could be launched any time soon. However, infrared is blocked by the atmosphere so an observatory without an atmosphere is required.

Yup. I don't know that I'd even call myself an amatuer astronomer but I remember being fascinated by a Nova episode about IRAS ages ago.

This is a very poorly explored region of the spectrum, hence the interest. I think the issue with sending up another Hubble is that it just isn't as much bang for buck.

Don't get me wrong - it seems silly not to have ONE visual spectrum space telescope, but looking into different wavelengths is far more likely to turn up revolutionary results and advance the field.

Here's an analogy. We discover a planet on a distant star. Which is more likely to turn up new results - a detailed observation of that distant planet, or a careful high-resolution analysis of craters on the Earth's moon? Sure, the latter might be good science, and turn up results, but it just isn't going to be as likely to change how we think about everything.

I'm only an amateur astronomer but... With adaptive optics we can get better visible light images with ground based telescopes like Keck than with any orbiting telecope that could be launched any time soon. However, infrared is blocked by the atmosphere so an observatory without an atmosphere is required.

Then imagine what we could do if we put an AO telescope array in space!

Adaptive optics help cancel out the distortions produced by the atmosphere. That's not particularly useful on a space telescope.

Once you've got adaptive optics to take away most of the biggest advantages for space telescopes, the ease of building giant mirrors on the ground takes over and you get much better performance for your budget.

Adaptive optics help cancel out the distortions produced by the atmosphere. That's not particularly useful on a space telescope.

Once you've got adaptive optics to take away most of the biggest advantages for space telescopes, the ease of building giant mirrors on the ground takes over and you get much better performance for your budget.

Depends on what your goal is. However, you are correct in this matter.

Yes, AO is generally a specific application of a telescopic array designed to thwart distortions caused by an atmosphere. I should have been a bit more clear on this. In this case, I was mixing up AO with a composite mirror/detector telescope.

However, imagine an array much larger than we could build on the ground. For instance, multiple telescopes in orbit around the moon, earth, and the sun? You could use that for all sorts of interesti

I thought you might be referring to interferometry in your original post, when you mentioned an array.

You CAN build very high resolution interferometers in space. There are some problems though. As far as I know you couldn't build a bigger visible or infrared interferometer than we can already build on the ground. The individual elements of those have to be linked by fibre optics because we don't have a good way of recording phase information for optical frequencies.

I thought you might be referring to interferometry in your original post, when you mentioned an array.

You CAN build very high resolution interferometers in space. There are some problems though. As far as I know you couldn't build a bigger visible or infrared interferometer than we can already build on the ground. The individual elements of those have to be linked by fibre optics because we don't have a good way of recording phase information for optical frequencies.

Space interferometers definitely work at radio frequencies, but they do different things than telescopes like Hubble. An interferometer has very high resolution (and very small field of view) but doesn't have matching light gathering ability. If you want to try to image extrasolar planets or count sunspots on other stars, an interferometer is the way to go. If you want to look at the early universe or do sky surveys, they are not.

As I recall, the communications issue is the reason we're still having trouble with getting interferometry working for visible-light wavelengths and shorter.

I thought you might be referring to interferometry in your original post, when you mentioned an array.

You CAN build very high resolution interferometers in space. There are some problems though. As far as I know you couldn't build a bigger visible or infrared interferometer than we can already build on the ground. The individual elements of those have to be linked by fibre optics because we don't have a good way of recording phase information for optical frequencies.

Space interferometers definitely work at radio frequencies, but they do different things than telescopes like Hubble. An interferometer has very high resolution (and very small field of view) but doesn't have matching light gathering ability. If you want to try to image extrasolar planets or count sunspots on other stars, an interferometer is the way to go. If you want to look at the early universe or do sky surveys, they are not.

As I recall, the communications issue is the reason we're still having trouble with getting interferometry working here on Earth for visible-light wavelengths and shorter.

It's because it makes sense to use space telescopes to look at radiation that can't be observed with ground based telescopes, because the Earth's atmosphere absorbs all of it. Herschel with its three instruments (HIFI, PACS and SPIRE) operates in the submm and far infrared, a part of the spectrum inaccessible from ground, and will spend a lot of observing time e.g. to look at interstellar water, a molecule believed to play an important role for the cooling of star forming clouds.

The way this is written and the story is written it makes the telescope sound like the next generation, bigger, better Hubble Space Telescope. That's not really that acurate. Hubble is designed to look primarily at visible light and near infrared. It also can observe in the UV or a combination of the spectrum using the instruments on board.
The Herschel telescope is designed entirely for infrared. It extends coverage below the capabilities of the HST's infrared camera/spectrometer package and has optics designed for optimal gain in the infrared.
Both of these kinds of telescopes have their advantages and limitations. Infrared alone won't allow for the kind of spectrometry and band analysis that Hubble is capable of. However, it will be able to resolve more distant objects, especially those obscured by dust or gas, much better than Hubble can and will be able to see things that the Hubble telescope can't. On the other hand, areas could be obscured if they have enough hot gas or if there are large medium temperature stars, like red giants and interstellar gas to reflect their light off of.
The reality is that both instruments fill an important role and that's why it's important to get the HST back up to its full capacity.

Arianespace Flight 188 will use an Ariane 5 rocket with an ECA upper stage to launch the European Space Agency's Herschel and Planck observatories. The Herschel infrared telescope will study the evolution of stars and galaxies and the Planck spacecraft will observe the cosmic background radiation left over from the Big Bang. [Jan. 14]

The ESA, NASA and JPL have collaborated on this project since this appears to scaled up version of Spitzer Space Telescope with 6 years of advanced technology and lesson learned from Spitzer Space Telescope.

Not fully. Spitzer is mostly a shorter wavelength (best at 1-25 microns) observatory than Herschel (best at 200-600 microns). You can understand vastly different things with that difference in wavelengths.